On the base of the measurement of 2 520 young oak trees 22 years old, the branchiness index, A SIX , was created as the ratio between the branch of the largest diameter and the tree’s di
Trang 1Original article
A simple tool to describe branchiness
in young deciduous forest stands
Gerhard Struck and Achim Dohrenbusch*
Institute of Silviculture, Department I: Silviculture of the Temperate Zones and Forest Ecology,
University of Göttingen, Büsgenweg 1, 37077 Göttingen, Germany (Received 6 September 1999; accepted 24 March 2000)
Abstract – Description of young tree’s quality is difficult, in particular there is no adequate and easy applied method to describe
branchiness The following contribution describes the development of a new index to estimate the branch’s diameter influence on tree quality On the base of the measurement of 2 520 young oak trees (22 years old), the branchiness index, A SIX , was created as the ratio between the branch of the largest diameter and the tree’s diameter at breast height (DBH) The idea was that a same diameter of branches is worse for small trees than for bigger ones In a very wide range of plant densities, no influences of the different initial densities on the A SIX values were found Trees in wider plantations with a higher branch diameter had as good A SIX -values than trees
in denser plantations with a lower branch diameter But, however, an influence of different site conditions on the A SIX was noticed.
A ISX was better within better site conditions compared to worse site conditions It is concluded that the description of branchiness using this ratio is better than the use of absolute diameter values A SIX integrates the stands growth potential in terms of DBH The use of this index avoids the overestimation of branch-thickness for trees with a high growth potential Due to a lack of literature dealing with branch-ratios, there are no comparisons to other indices in the discussion.
branchiness / index / stand density / deciduous forests / quality / Quercus
Résumé – Développement d’un nouvel indice de branchaison « ASIX » : un outil simple pour décrire la branchaison dans de jeunes peuplements de forêts décidues La description de la qualité de jeunes arbres est difficile, en particulier il n’existe pas de
méthode facile et adéquate pour décrire la branchaison La contribution suivante décrit le développement d’un nouvel indice pour estimer l’influence du diamètre des branches sur la qualité de l’arbre Sur la base de mesures de 2 520 jeunes chênes (âgés de 22 ans), l’indice de branchaison, « ASIX », a été créé comme étant le rapport entre le diamètre de la branche la plus grosse et le diamètre à hauteur de poitrine (DBH) L’idée est que, à diamètre identique, les arbres les plus petits sont les moins bons La densité initiale du peuplement n’a pas d’influence sur les valeurs calculées de l’indice ASIX Les arbres des plantations les moins denses, avec des dia-mètres de branches plus élevés, ont une valeur de l’indice aussi bonne que celle des plantations moins denses avec des diadia-mètres de branches inférieurs Les conditions du milieu influent cependant sur la valeur de cet indice ASIX est meilleur lorsque les conditions stationnelles sont bonnes On conclut que la description de la branchaison par cet indice est meilleure que l’usage de la valeur abso-lue du diamètre ASIX intègre le potentiel de croissance en terme de DBH L’usage de cet indice évite les surestimations de la taille des branches sur les arbres à fort potentiel de croissance En raison du manque de bibliographie dans ce domaine d’indice rapport de branche, il n’y a pas de comparaison avec d’autres indices dans la discussion.
branchaison / indice / densité du peuplement / forêts décidues / qualité / Quercus
* Correspondence and reprints
Tel (49) 551 39 36 78; Fax (49) 551 39 21 70; e-mail: adohren@gwdg.de
Trang 2G Struck and A Dohrenbusch 812
1 INTRODUCTION
The objective description of young tree’s quality in
deciduous forests is still difficult, in particular within
forest investigations [1, 28] Tree quality is often
esti-mated by the use of categorical variables [6, 8, 9, 24,
28]
One main factor influencing the trees’ quality is the
diameter of branches This feature is mainly assessed by
the measurement of absolute branch diameter [7, 10, 16,
21, 25, 26, 27] But the comparison of absolute branch
thickness of differently treated stands does not consider
the growth potential of the trees Harmer[14]points out
that vigorous trees often carry heavy stem branches that
reduce timber quality, whereas less vigorous trees have
smaller branches but are often of bad growth
But does a branch of certain diameter influence the
tree quality of good and bad growing trees in the same
way? Certainly not Due to this, there is a need of easily
to measure parameters to describe the branch-diameter
effect on tree quality under consideration of the
individ-ual growth potential
Following the ideas of Kramer [17] and Mayer [20],
which created different crown-indices, the following
arti-cle describes the development of a branchiness index
(ASIX) on interval scale
This could be a method to describe the influence of
branches on tree quality under consideration of the
indi-vidual growth potential of each tree
2 MATERIALS AND METHODS
Base of this investigation are 2 520 22-year-old oaks,
which were measured on 24 experimental plots located
in the forest district Fuhrberg near Hannover [28] This
region in north-western Germany is characterised by a
subatlantic climate with an annual precipitation of
700 mm and a mean annual temperature of 14.5 °C The
soil is a poor sandy pleistocene podsolic cambisol
(FAO) The plots have been afforested in 1976 using
three different densities and two year old seedlings The
oaks were 24 years old at the date of measurement The
three different initial densities were very high compared
to actual plantation densities The current density within the stand is still very high and the differences between
the initial densities are also obvious (table I).
Before afforestation, the whole logging slash includ-ing the humus layer was pushed on ramparts usinclud-ing bull-dozers Due to this practice a gradient of different nutri-ent situations can be noticed on the plots The growth potential of trees was considerably higher close to the ramparts compared to potential of plants in a greater dis-tance [29] This experimental design was very suitable to investigate branchiness of young trees under different yield situations
To detect the effects of initial density and logging slash removal, the following features were measured on
105 trees [28, 29, 30]:
– Tree height;
– DBH;
– Branch of the largest diameter within in the first 2.5 m height Branch thickness was measured, at right angles to the branch, 2 cm apart from the stem-inser-tion with digital slide-gauge Living branches only were considered
3 RESULTS
Tree height showed a reaction to the nutrition gradi-ent In the “wide spaced” plantations, trees in a distance
of 12 m to the humus ramparts reached only 75% of the height of trees directly beside the ramparts In the
“medi-um spaced” and “narrow spaced” plantations, the height growth decreased to about 50% compared to the trees close to the ramparts The rampart’s effect on the tree
height was greater, with higher plant-density (figure 1).
The DBH decreased for all initial densities, with a dis-tance of 6 m from the rampart As already mentioned for height, the effect on the DBH increased with higher plantation density Compared to the height, DBH was
affected less Figure 2 furthermore shows that mean
DBH was greatest for the “wide spaced” plantation
Table I Different initial densities of the experimental plots.
Trang 3followed by the “medium” and finally the “narrow
spaced” plantation
As a result of less competition, the growth potential of
individual trees was higher for the “wide spaced”
planta-tion
In the “wide space” plantation, the branch thickness
was greater compared to the other densities Branches in
the wide spaced plantations had a mean diameter of about 1.5 cm, whereas the branch-diameter in the
medi-um spaced plantation (1.3 cm) did not differ significantly from the narrow spaced plantation (1.2 cm)
Within the same plantation density, the branch thick-ness was equal even in different distances to the humus rampart Trees with a low height and a low DBH had
Figure 1 Effect of the humus
rampart distance on the mean height in the different initial densities.
Figure 2 Effect of the humus
rampart distance on the mean DBH for the different initial densities.
Trang 4G Struck and A Dohrenbusch 814
branches as thick as good growing trees close to the
ram-parts Hence, the nutrition gradient seemed not to
influ-ence the branch diameter Struck [28] showed that
nei-ther the mean branch diameters were influenced by the
rampart distance, nor the extreme values He found as
heavy branches near the ramparts as in a distance of
more than 20 m (figure 3).
A significant correlation between branch thickness of
the biggest branch and the DBH (r = 0.5) was found But
the value dispersion was to high to create a successful
regression between the parameters
This result pointed out that, on the one hand, there
were small trees with comparable big branches and on
the other side large trees with comparable small
branches
Assuming that the quality of a good growing tree, in
comparison to a bad growing tree, is affected less by a
branch of equal thickness, the growth potential has to be
integrated in the model to describe the effect of
branchi-ness on tree quality The branchibranchi-ness index (ASIX) was
accordingly calculated for each single treeias follows:
The ASIX-value describes the branch thickness of a tree
as the percentage of its DBH Using this method, the
description of branch thickness is weighted with the
growth potential The tree’s quality decreases with an increasing ASIX-value
dependence on the ramparts distance exemplary for the
“narrow spaced” plantation
The branches grew thicker in relation to the DBH with a growing rampart distance Therefore, tree quality decreases
If the different plantation densities were compared on the base of the ASIX, a different result is found compared
to the analyses of absolute branch thickness The branch-iness indexes of the different initial densities do not dif-fer significantly The distributions of the values are almost identical as well Merely the “wide spaced” plan-tation shows a smaller value-dispersion
Even, the median value shows that tree quality, mea-sured by the ASIX value, for the “narrow spaced” plan-tation is worse than for the other densities This is the result of bad DHB-growth on the “narrow spaced” plan-tation It is conspicuous that the wide space plantation does not reach ASIX-values of more than 85% Due to the good DBH-growth the tree quality for this plantation density is affected less by the absolute thicker branches
(figure 5)
Mean values of the whole collective are of less inter-est for the inter-estimation of the young stand’s quality The future economical value of the young stands is formed
by the future crop trees These trees will be normally
Figure 3 Effect of the humus
rampart distance on branch thickness for the different initial densities.
Trang 5selected from the 20% biggest trees of a stand Cause our
measurement included 840 trees per different density,
the following results refer to 170 future crop trees in
each density
It is obvious considering this collective, that the ASIX
is slightly better for the “narrow spaced” plantation
com-pared to the other densities (figure 6).
Figure 4 Effect of the
nutri-ent gradinutri-ent on the develop-ment of branchiness index (narrow spaced plantation).
Figure 5 Box Wisker plot
for the branchiness index of the different densities.
Trang 6G Struck and A Dohrenbusch 816
Using this index and considering a certain value for
the ASIX, it is shown how the branch thickness could
increase without detecting a decrease in quality
Therefore the equation for the ASIX-calculation is
changed to:
maximum branch thicknessi(cm) =ASIX×DBHi(cm)
100
If the silvicultural aim is an ASIX of up to 50%, the
branch thickness should have been increased from
2.54 cm in the “narrow plantation” to 3.47 cm in the
“wide plantation” without any decrease in tree quality
(table II).
If the silvicultural aim is a certain maximum ASIX, the
mean increment of branch thickness without a loss of
future tree quality, can be calculated The ASIXis able to
quantify the influences of different densities on the branch quality under consideration of the stand growth potential The results show that the calculation of the
ASIX could be method to detect influences of thinning age and thinning grade on tree quality This application
of ASIX should be tested in further investigations The
ASIX-method enables the user to note relative changes in branch quality
4 DISCUSSION
Unbiased estimation of quality in young deciduous forest stands is a problem In practice, the trees are selected by individual preferences of foresters But sci-ence, as well, lacks objective and easy methods to esti-mate tree quality Hakkila [12] and Hakkila et al [13] used similar indices to ASIX to estimate branch amount
Figure 6 Box Wisker plot
for the branchiness index in the different densities for the 20% biggest trees.
Table II Calculation of branch thickness with a special Asix.
Trang 7as a raw material source But we found no similar
quo-tients to ASIX with the aim to describe tree quality
Categorical variables have been mainly used [1, 7, 9,
28] Hence, the comparison of different investigations
using categorical features is difficult, because the
estima-tion depends on subjective impressions [9] The results
differ from person to person even if the same stands are
surveyed These kind of disadvantages caused by using
categorical variables can be avoided by the use of
fea-tures on interval scales These interval scale variables
must be created so that the results are independent from
the measuring person For this reason, forest scientists
prefer the comparison of absolute branch thickness The
disadvantage of this method is that the different yield
potentials of the stands cannot be considered
It is pointed out in many investigations, that trees,
which grew in “wide spaced” plantations are of worse
quality than trees growing in dense stands, because
branch thickness increases with an increase in individual
space [7, 10, 16, 21, 24, 25, 26, 27, 28] The current
investigation showed the same results:
The oaks’ branches were thicker with decreasing
ini-tial density but not thicker with worse site conditions
[28, 29, 30] The site gradient effects are discussed by
Struck [28] He did not find any influences on branch
architecture Large branches were found in each different
distance to the humus ramparts
However, it should be considered that plants having
more space to grow have better DBH growth [2, 10, 11,
17, 24, 26]
The rare at which branches are occluded is also
important in determining wood quality In most cases,
branches will be overgrown faster in wide space
planta-tions Niemisto [23]reports that the self–pruning process
of dead branches may be slowed down in very dense
stands Becker et al [2] point out that thicker branches
have no influences on the quality, if these branches
would be overgrown faster, and the better youth growth
leads to the development of relatively fine wood
Grünebaum et al [11] investigated oaks from a wide
space plantation and noticed that the faster youth growth
did not lead to worse inner wood quality It is therefore
clear that an analysis of the absolute branch thickness,
only, in differently treated stands is not enough to
esti-mate their future quality Scientists do not know which
supposed tree defects will disappear in the stand’s future
life The estimation of branchiness and its effect in later
ages is difficult
In the literature one can find a large amount of models
dealing with branchiness (for example, [3, 4, 5, 14, 18,
19, 22]) Mainly, the intention of such investigations is
the prediction of the development of architecture under
different silvicultural systems and they deal with conifer-ous species The development of ASIX had not the aim
to create another architectural model The ASIX values are proposed as an additional, very easy, tool to describe and compare tree quality The periodical comparison of the ASIX-values is able to detect the influences of differ-ent silvicultural treatmdiffer-ents on the branch quality It is clear, that the diameter of the largest branch is not the only factor responsible for the future value of a tree Amount of branches on the trunk is also very important
It is possible to create other indices on the base of
ASIXto avoid these problems: For example, it is possible not only to use the diameter of the largest branch but also to calculate a mean value of several branches It is furthermore conceivable to weigh the ASIX-value with the number of branches on a special bole length A tree with a high number of little branches does therefore not automatically have a better quality than a tree with only one or two big branches The emphasis on a certain bole length further integrates the fact that branches in a spe-cial insertion height (for example more than 10 m) did not influence the quality at all
The ASIXis only an attempt to define a value, which allows an objective estimation of quality in young decid-uous stands Naturally, appearing branches and their thickness are an important quality defect if the trees should be sold The estimation of tree quality using the
ASIXin old forests is less successful because branches on trunks to be sold are always defects and independent from the trees DBH The use of the ASIXshould be con-centrated on young stands up to polewood age The expected branch-free bole length should be reached at this age and the stem defects of the young stands should
be obscured by growth, in particular for the crop trees In most cases, silvicultural goal is a certain branch free bole length Branches inserting higher than this length are of less interest Trees with a good growth potential should reach this silvicultural aim in a shorter time, even if the branches grew thicker Asix-values may help to avoid the overestimation of branchiness for the future quality development, because this index integrates the growth potential
The ASIXis not able to compensate the measurement
of branch diameters, but the comparison of differently treated stands is easier using this method It is very important that quality described by the ASIXis not only a value for absolute branch diameters but considers the individual tree growth potential
Trang 8G Struck and A Dohrenbusch 818
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